Metallurgical thermocouple

ABSTRACT

This invention concerns a thermocouple which is protected by an outer sheath comprising inner and outer tubes with the annular space filled with low temperature sintering refractory material which is preferably beaded before filling in the space and in which the outer tube is constricted to compact the material. The refractory material will include borosilicate and boric acid powder.

FIELD OF THE INVENTION

This invention relates to thermocouples and more particularly tothermocouples for use in determining the temperature of molten metals.

BACKGROUND TO THE INVENTION

Many kinds of thermocouples have been designed and used for use in themetallurgical industry. In general in the melting and casting processesfor the production of primary and secondary aluminium the use of socalled “Marshall Tip Thermocouple” has become fairly standard practice.In the baking process of carbon anodes for the production of aluminiumthe use of wire and bead or mineral insulated thermocouples protected bysuitable metallic sheaths has become the norm.

In the ferrous metal industry platinum rhodium type thermocouples areused because the temperatures of molten steel are generally greater thanthose at which some of the components of the thermocouple used in thenon-ferrous industry melt. It is difficult to provide insulation of theplatinum rhodium element at molten steel temperatures for any length oftime and insulation which will enable repeated use of the thermocoupleis also difficult to provide. Consequently in the interests of economy,the thermocouple for this industry has been designed to protect thethermoelement for a maximum of about 4 seconds of immersion time, whichis sufficient to obtain a single measurement.

Such thermocouples incorporate the smallest amount of the requiredmaterials, and where practical, the lowest cost materials in order torender the device expendable after only a single immersion into molteniron or steel.

Thus with an overriding cost consideration different thermocouples havebeen developed to meet the particular requirements of particularapplications.

There remains however the basic requirements for all thermocouples whichis the integrity of the temperature measurement obtained. To achievethis it is necessary that the measuring probe be protected againstelectrical conductivity of its immediate protection material and furtherthat a barrier of sufficient mechanical strength be provided against theinherently corrosive attack from the in situ environment in which thethermocouple is to be used.

OBJECT OF THE INVENTION

As stated the kind of thermocouple used in any application is driven bycost effectiveness. All of the thermocouples referred to suffer some orother disadvantage as a result of cost and it is the object of thepresent invention to provide a thermocouple which with minormodification can be used in the ferrous and non-ferrous industries andwhich can be made at a high cost effectiveness.

SUMMARY OF THE INVENTION

According to this invention there is provided a thermocouple comprisinga sensing tip and electrical connection with a mineral insulatedthermocouple cable characterised in that the shielding is provided by alow temperature sintering refractory material.

Further features of this invention provide for the thermocouple cable tobe types K and N for non-ferrous metals or type W, W3, W5 and molybdenumrhenium for ferrous metals.

Still further features of this invention provide for the shielding to bein the form of a sheath having inner and outer metal tubes over a fillerof low temperature sintering refractory material and for the tubes to bedrawn down, swaged or rolled to compact the filler between them and fora conventional binder material to be added to the refractory material togive it the required green strength when the refractory material isbeaded before introduction between the tubes.

The invention also provides for the refractory material to includeparticulate borosilicate and boric acid powder, for the borosilicate tocomprise between 6% by weight of the total refractory material, for theboric acid to comprise about 3% to 5% by weight of the total refractorymaterial and for the boric acid content of the refractory material to beabout one half that of the borosilicate content.

Still further features of this invention provide for the inner and outertubes of the sheath to be of stainless steel.

The invention provides a method of shielding a thermocouple comprisinglocating beads of suitably bound refractory material between an innermetal tube and an outer metal tube and reducing the sheath down to apredetermined size by drawing swaging or rolling during which processthe beaded refractory material is compacted between the inner tube andthe outer tube.

A further feature of this method provides for the reduced sheath to besubsequently annealed and the refractory material to be at leastpartially sintered simultaneously with the annealing of the sheath.

Yet further features of this invention provide for the tip to beprovided by the dissimilar metal wires of the mineral insulatedthermocouple cables providing a hot junction for the thermocouple withthe wires embedded in magnesium oxide and this latter supported by asheath as above defined or by a tube of the same metal as one wire ofthe cable housing the other wire of the cable to form the thermocoupletip with the wire embedded in a low sintering refractory material.

It is to be understood that where reference is made to metal tubes orwires of thermocouple cable materials being negative and positive Type Kor Type W metals this polarity may be reversed. Further the terms“shield” and “shielding” are used to signify both thermal and electricalinsulation.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of this invention is described below withreference to the accompanying drawing in which

FIG. 1 is an oblique view of one form of thermocouple; and

FIG. 2 is a section through the tip of an alternative form ofthermocouple.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

As illustrated the thermocouple (1) is made from a length ofconventional mineral insulated thermocouple cable (2). This comprises anouter casing (3) of stainless steel around a magnesium oxide insulatingbody (4). The thermocouple is a Type K thermocouple connected to anoperating tip (5) having the negative wire welded to a negative Type Ktube (6) which extends around the extended Type K positive wire (7). Alow temperature sintering refractory material (8) is packed in the tube(6) around the wire (7).

As mentioned above the Type K tube may form the positive connection andthe wire the negative connection for the thermocouple tip.

The connection between the mineral insulated thermocouple cable (2) andthe thermocouple tip (5) is located within a suitable metallicoversleeve (8) having further low sintering refractory insulation (9)around the cable (2) and tip (5) and within the oversleeve (8).

To provide sufficient physical strength as well as further insulation asheath (10) having inner and outer tubes (11) and (12) is provided. Theannular space between tubes (11) and (12) is first packed with beadedlow temperature sintering refractory (13). The outer tube is then drawndown over the refractory material crushing the beads to reduce theporosity and also increasing the physical green strength of the sheath.

The outer tube may be annealed after the drawing operation and therefractory material at least partially sintered during this annealingprocess. It has been found advantageous to pre-dry the refractorymaterial before use to a temperature of between 135° and 150° C.

This material may be sintered during in situ use but is preferably atleast partially presintered by heating before use and maintained underconditions mitigating the ingress of moisture.

It has been found that a very suitable refractory material can beobtained by the addition of crushed borosilicate and boric acid powderin a proportion by weight of about 2 to 1 to any refractory material.Preferably the borosilicate will comprise between 6% and 10% of theweight of the composite refractory material, most preferably about 8%.The applicant has found that this mixture precipitates a reaction atonly ±780° C. which is very similar to that of conventional sintering inwhich the surface of the aggregate particles soften and the particlesfuse together to form a more dense mass. It is assumed that theborosilicate, which has a melting point of about 780° C., provides thesoft surface on each particle, but only in the presence of boric acid.Once formed, it no longer melts at the same temperature. The result is adense body that does not lose its bond even at temperatures in excess of1000° C.

Once sintered, the ingress of atmospheric moisture, which must occurbecause it is still porous, appears to have no effect on the electricalconductivity of such a body. This is a decided advantage in themanufacture of thermocouples where the electrical insulation ofconductors at elevated temperatures has always represented a significantproblem.

This thermocouple responds instantly when immersed into molten metal, orany electrically conductive compound which bridges the two conductorends. This provides temperature measurement of liquids as well as forsolids to be obtained with the same thermocouple.

It will be appreciated that the thermocouple can be made in anacceptably cost effective manner.

The thermocouple above described is that which will be used fornon-ferrous metals. For ferrous metals the Type W or molybdenum rheniumthermocouple cable and tip will be used.

FIG. 2 illustrates an alternative form of thermocouple.

In this form the hot junction (15) is formed by baring the ends of thewires (16) (17) of the thermocouple and fitting a cap (18) over thisjunction. The end of the cable is inserted through the sheath (19) whichhas its end shaped to close together to form an outer cap (20) from theouter tube, a continuous layer of low temperature sintering refractorymaterial and the closed inner tube.

This thermocouple may be satisfactorily used where instantaneoustemperatures are not necessary and they can be used to obtain continuoustemperature measurements. Even should the outer tube become erodedshielding is still afforded by the sintered refractory material.

Thermocouples for both ferrous and non-ferrous material can thus beprovided with a high degree of shielding for both instantaneous andcontinuous temperature recordings.

What is claimed is:
 1. A thermocouple comprising a sensing tip inelectrical connection with a mineral insulated thermocouple cablecharacterized in that additional external shielding is provided by a lowtemperature sintering refractory material including particulateborosilicate and boric acid power, in which the tip is formed from athermocouple cable with a negative metal tube housing a positive wireembedded in the low temperature sintering material.
 2. A thermocouplecomprising a sensing tip in electrical connection with a mineralinsulated thermocouple cable characterized in that additional externalshielding is provided by a low temperature sintering refractory materialincluding particulate borosilicate and boric acid power, the shieldingis in the form of a sheath having inner and outer metal tubesconstricted over a filler of low temperature sintering refractorymaterial, in which the tip is formed by providing a hot junction fromthe wires of the thermocouple cable and supported by the sheath withboth tubes and the refractory material formed to cap the hot junction.